Because of wear and other effects, rotors, or rotating members such as fans, turbines, shafts, and motors, must be periodically re-balanced so that they operate more efficiently. As a part of this balancing procedure, the rotor is analyzed to determine where it is out of balance, i.e.--heavy spots are located, and then counterbalancing weight is added to the rotor to restore its balance.
Typically, the rotor to be balanced must be taken out of service, as it must be stopped and started for several different test runs. This requirement means that technicians are interested in balancing instruments that can perform the necessary tests and analyses in as little time as possible so that the rotor can be brought back on line as soon as possible. The shorter the time that the rotor is down for maintenance, the lower the costs associated with the balancing procedure.
To balance a machine, typically, the rotor is stopped and vibration sensors are attached to the rotor at each bearing location, on the outside of the bearing mounts on stationary, non-rotating sites., Usually a mark is placed on the rotor so that there is a reference for determining where the heavy spot on the rotor is located and a stationary rotation sensor is used to detect the passing of the reference mark as the rotor rotates.
A reference run is then performed by bringing the rotor up to operating speed. The technician collects data from the vibration sensors, and correlates it against the data collected from the rotation sensor.
Next, trial runs are performed, one trial run for each weight plane on the rotor. For each trial run, a known amount of weight is placed in a known location on a single weight plane. The rotor is brought up to speed, and data is collected as during the reference run. The rotor is then stopped and weight is placed on the second weight plane. The rotor is brought up to speed, and data is again collected. This is repeated for each weight plane on the rotor.
When the trial runs are completed, the technician calculates influence coefficients using the data sets collected from the reference and trial runs. The influence coefficients are used to calculate the amount and location of counterbalancing weight to be added to the light spot in each weight plane of the rotor to bring it into balance. The location of the weight to be added is expressed in units of degrees away from the reference mark that was placed on the rotor. An angle sensor, attached to an axial face or appendage of the rotor, is used to determine the correct rotor orientation. The technician places the weight, and performs another run to verify that the rotor has been brought within allowable balance tolerances.
Trim runs using the same influence coefficients are performed if the rotor is not quite within tolerance. If the rotor had been far out of balance before the start of the balancing procedure, it may be necessary to perform a second set of reference and trial runs with the balancing weight in place, to determine new influence coefficients.
There have traditionally been several problems associated with balancing procedures, all of which result in higher maintenance costs than are necessary because they cause the procedure to take longer than is needed, thus increasing the cost associated with paying the technician, and the cost of having the machine out of service for an extended length of time, as well as causing unnecessary wear on the machine.
The influence coefficients for a rotor do not need to be calculated anew each time the rotor is balanced, if it is possible for the technician to set up the vibration and rotation sensors in exactly the same manner each time the balancing procedure is performed. Unfortunately, notes taken by different technicians, who make notations with differing degrees of accuracy and descriptiveness, have proven inadequate to ensure repeatable equipment set-up. There is a need, therefore, for an instrument that ensures that the same equipment set-up is accomplished each time the rotor is balanced, even if it is not the same technician that performs the balancing procedure each time.
The technician collects data from the vibration sensors, the rotation sensor, and the angle sensor during the balancing procedure. This data must all be correctly correlated to properly balance the rotor. Any data errors, such as reading a sensor incorrectly, recording the data incorrectly, or mismatching data taken from different sensor locations will result in an incorrect amount of weight being applied in an incorrect location on the rotor. The rotor will not be balanced, and the time required for the procedure will be unnecessarily extended. There is a need for an instrument that ensures that the data collected is accurate, timely, and properly correlated with all other data.
Even though a rotor exhibits excessive vibration, the problem may not be rotor unbalance. Rotor misalignment, resonance, cracked or worn bearings, or some other problem may be responsible for the vibration. Performing a balancing procedure will not fix any of these problems, but will extend the length of time the rotor is down for service. An instrument is needed to quickly determine if a problem other than unbalance is present, so that the technician is alerted to the true nature of the problem, and down-time is kept to a minimum.
The vibration, rotation, and angle information received from the sensors is in a "raw" form that is difficult to interpret. This difficulty leads to mistakes which extend down-time. There is a need for an instrument that presents the data to the technician in a form that is quickly and easily understood, and allows for immediate redisplay according to the technician's perspective and preference.
It takes a variable length of time for a rotor to reach a steady state of operation once started and turning at full speed, due to thermal and other effects. There may also be other conditions which either prevent the rotor from ever achieving steady-state operation, or intermittently affect steady-state operation. Collecting non-steady-state data will result in erroneous results as mentioned above, and extended down-time. An instrument is needed that consistently determines when the rotor is at steady-state, and when reliable data is being gathered.
Because the balancing procedure is lengthy, complicated, varies according to rotor configuration, and must be performed in a specific sequence, there is ample opportunity to confuse the order of steps to be performed. When this occurs, the technician must return to the last step performed in proper sequence, and restart the procedure from that point. An instrument for ensuring that the proper steps in the proper sequence are always followed is needed.